The invention relates to a method for controlling a crane, the method comprising giving velocity requests as control sequences from a crane control system to crane drives and reading and storing the velocity requests in a control system, whereby each velocity request is compared with the previous velocity request and, if the velocity request is changed, an acceleration sequence for the corresponding velocity change is formed and stored, after which, irrespective of whether the velocity request has changed, summing the velocity changes defined by the stored acceleration sequences at a particular time and adding the obtained sum to the previous velocity request to achieve a new velocity request, which is set as a new control and velocity request for the crane drives, and performing some of the velocity changes defined by the summed acceleration sequences at the definition time of each selected sequence on each program round, i.e. control step (sample interval) and performing the rest of them as delayed.
The above method is disclosed in Finnish Patent 89155. By using this method it is possible to efficiently prevent the undesired swinging of load fastened to the crane, disturbing the use and operability of the crane when the crane is controlled and the load is transferred. The method improves the properties of a crane control system by summing, in a particular manner, different control sequences eliminating the swinging occurring after load acceleration. By using this method, the end velocities forming the target of acceleration can be randomly changed at any time, also during the actual velocity change sequences, and a new, desired end velocity is achieved without undesired swinging of the load.
According to prior art, a control preventing the load swinging typically comprises two acceleration sequences, the time difference of which is half of the oscillation time of the load. Another, easily definable control consists of three acceleration sequences with the same magnitude but varying directions, the first sequence being positive, the second negative and the third positive, whereby the time between the sequences equals to one sixth of the oscillation time of the load. In the method of Finnish Patent 89155, these control sequences preventing the load swinging can differ from each other and an unlimited amount of them can be defined. It is essential that when the accelerations defined by them are summed up, a control preventing the swinging is achieved. When the sum of the accelerations is selected in such a manner that it implements the desired velocity change, a control is achieved, wherein the desired end velocity of the crane is produced without swinging of the load.
U.S. Pat. No. 5,526,946 discloses an application of the same subject, whereby, whenever the reference value of velocity changes, a half of it is performed and the other half is stored in a table, where the performance of it is delayed by a half of the oscillation time of the load. This is a preferred embodiment of the method according to Finnish Patent 89155 and used in computer calculation.
When a new control preventing swinging is calculated on each program round, i.e. control step, and, on the other hand, the controls stored in the tables are updated and the control formed by them is also calculated on each program round, there will be calculatory problems. When the calculation is accelerated, the size of the tables becomes bigger and more calculations must be made on each program round, because the size of the table is defined on the basis of the relation between oscillation time and the control step. When the control step is decreased, for instance, from 100 milliseconds to 10 milliseconds, the number of calculations will be decupled. As the pendulum arm of the load becomes longer, the number of elements to be stored will grow further. To decrease the control step is reasonable, because it shortens the reaction delay and thus provides the crane driver with a better control of the driving.
Electric drives, which are used for controlling the velocity of traversing motors of a crane, are controlled by a microprocessor and the duration of their programs is short, 2 to 5 milliseconds. As to the technical implementation, it is also preferable to calculate the velocity request preventing the swinging in the same time plane. As can be seen from the above, as the control step becomes smaller, the amount of memory capacity and the number of calculations increase rapidly. In some cases, this makes it more difficult to calculate the preventing of the swinging when, for instance, electric drives controlling the rotational speed of traversing motors are used.